Polyols are very well known to those skilled in the art due to their utility as reactants for the preparation of a variety of polymeric or resin compositions. Typical reactions include curing or crosslinking reactions with polyisocyanate materials having at least two isocyanate groups per mole, melamines or formaldehyde resins, as well as esterification reactions with unsaturated monobasic fatty acids to form alkyl resins.
One class of polyols includes polyester polyols or the hydroxy terminated polyesters. These are generally prepared by an esterification reaction of a diol or mixtures of diols and triols with a dicarboxylic acid or anhydride. Examples of suitable diols and triols are ethylene glycol, propylene glycol, 1,3 propane diol, 1,4 butane diol, neopentyl glycol, trimethylol propane and the like. Examples of such dicarboxylic acids and anhydrides are phthalic acid, phthalic anhydride, isophthalic acid, maleic acid, maleic anhydride, succinic acid, adipic acid and the like. These polyester polyols, however, are generally high viscosity resins which have to be diluted or dissolved in relatively large amounts of a suitable solvent in order to provide low viscosity, easy to apply coating compositions when mixed with a curing or crosslinking agent.
Because a large amount of solvent is required to reduce the viscosity of these polyols, they are not suitable for the formulation of high solids coatings. Also, the issuance of recently strengthened EPA (i.e., Environmental Protection Administration) regulations as well as the high cost of such solvents are forcing end users to significantly reduce the level of solvent emissions from their operations. Moreover, due to the presence of the interfering solvent, these polyols cannot be used in compounds for potting or molding applications.
Castor oil is a triglyceride ester of ricinoleic acid which contains approximately 3 hydroxyl groups per molecule. It is, therefore, a polyester polyol which can be reacted with polyisocyanates, melamine and formaldehyde resins, or used in the formation of alkyd resins. These castor oil compositions, however, have relatively poor mechanical properties and limited solvent resistance.
A second class of polyols is the acrylic polyols, which are prepared by the copolymerization of a hydroxy acrylate or methacrylate with acrylate and/or methacrylate esters or styrene. Examples of suitable monomers are hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, butyl methacrylate and the like.
These hydroxy-containing acrylic polymers, like the polyester polyols, are high viscosity compounds and require large amounts of solvent to provide low viscosity, easy to apply coating compositions when mixed with the desired curing or crosslinking agent. Again, due to the presence of a large amount of solvent, these polyols are also not suitable for use in high solids coatings, or for potting and molding applications.
A third class of polyols comprises polyether polyols which are prepared by the homopolymerization and copolymerization of ethylene oxide and propylene oxide. Although some of the polyols in this class have acceptable viscosity, particularly those with low molecular weights, they are not suitable for high solids coating applications because of their poor light stability, poor weathering properties and limited solvent and water resistance.
Resin compositions which contain hydroxyl groups, such as alkyd resins, are also useful as polyols for coating compositions. These resins are produced as products of an esterification reaction involving a polyhydric alcohol, i.e., a polyol such as glycerol, sorbitol, ethylene glycol or pentaerythritol, and a monobasic fatty acid, most of which are derived from natural drying and nondrying oils, such as linseed oil, soybean oil and castor oil, to form resin-based coating compositions having acceptable properties for certain applications. The resin may then optionally be further modified by the addition of a polybasic acid composition.
The reaction of hydroxyl groups with isocyanates to form urethanes is very well known. A large number of polyols and a large number of polyisocyanates are available to form polyurethane compositions useful as adhesives, potting compounds and coatings. However, due to the recent EPA regulations and the high cost of organic solvents discussed above, these industries have been forced to significantly reduce the level of solvent emission from their operations.
One approach to meeting the new EPA regulations is the utilization of high solids compositions. In the coating industry, for example, high solids coatings are generally defined as having a non volatile content of approximately 80% or a VOC, i.e., volatile organic content, of 2.8 pounds per gallon or less. The conventional hydroxyl terminated polyesters, the copolymers of hydroxy functional acrylate and methacrylate with acrylate and methacrylate esters and styrene, the copolymers of allyl alcohol and other unsaturated monomers such as styrene, and hydroxyl-containing alkyd resins cannot be used in the high solids coatings because of their high solvent requirements for preparing low viscosity solutions. Such low viscosity solutions are essential for the preparation of coatings with good atomization, leveling and flow out properties.
The reduction in the molecular weight of these resins could result in higher solids and lower viscosity solutions, but this approach has the disadvantage of producing volatile oligomers and low equivalent weight resins. The oligomers could volatilize during baking and adversely affect the solid content and the VOC of the coating. The low equivalent weight resins require larger amounts of isocyanate for curing and, therefore, their presence adds considerably to the cost of the finished coating. The most expensive ingredient in a polyurethane composition is the isocyanate and especially the aliphatic isocyanates which, because of their superior UV stability, are preferred in high solids applications.
Low molecular weight, low viscosity isocyanates could be used with some advantage in lowering the viscosity of the final compositions. This approach, however, is not acceptable because the low molecular weight isocyanates have a vapor pressure which is too high and they are known to induce respiratory and skin problems to the user as well. Therefore, the coating manufacturers have adopted the practice of using isocyanate prepolymers and adducts which are much safer to use, but have much higher viscosities than the corresponding monomeric isocyanates.